1. Field of the Invention
The present invention relates to a display device using a semiconductor element (an element including a semiconductor thin film). In particular, the present invention relates to an active matrix type display device using an electroluminescence (Electro Luminescence: EL) element and a thin film transistor (hereinafter, referred to as “TFT”). Furthermore, the present invention relates to an electronic appliance in which a display device is used in its display portion.
2. Description of the Related Art
A so-called self-light-emitting display device in which a pixel is formed using a light-emitting element such as a light-emitting diode (LED) has received attention in recent years. As the light-emitting element used in such a self-light-emitting display device, an organic light-emitting diode (OLED, also referred to as an organic EL element, an electroluminescence (Electro Luminescence: EL) element, or the like; hereinafter, referred to as an EL element in this specification) has attracted attention, and is used for an organic EL display, and the like.
Since an EL element is a self-light-emitting element which has an electroluminescent layer between a pair of electrodes and which emits light by current flown between the electrodes, it has advantages over a liquid crystal display that visibility of a pixel is high, backlight is not necessary, and response speed is high. Luminance of the light-emitting element is controlled by a current value flowing to the light-emitting element.
The light-emitting element has properties in which a resistance value (internal resistance value) is changed by an environmental temperature (hereinafter, referred to as ambient temperature). In particular, when a room temperature is regarded as a normal temperature, the resistance value is decreased as the temperature becomes higher than the normal temperature, and the resistance value is increased as the temperature becomes lower than the normal temperature. Therefore, in a constant voltage drive, when the temperature becomes high, a current value is increased and luminance higher than desired luminance is obtained, whereas when the temperature becomes low, a current value is decreased and luminance lower than desired luminance is obtained. The light-emitting element which has been used recent years has properties in which a current value is decreased with time even if a predetermined voltage is applied.
Due to the properties of the light-emitting element mentioned above, variation in luminance is generated when the ambient temperature is changed or change of a current over time is occurred. In order to solve the problem of luminance variation in a light-emitting element due to the ambient temperature change and change of a current over time, it is proposed to provide a monitor element (for example, refer to patent document 1: Japanese Patent No. 3696116). One of electrodes of the monitor element is connected to a constant-current source and the input (an input terminal) of an amplifier circuit, while the output (the output terminal) of the amplifier circuit is connected to one of the electrodes of a light-emitting element provided in a pixel in a pixel portion. According to such a structure, the current flowing through the light-emitting element of the pixel is kept constant based on the temperature characteristic of the monitor element. In this specification, “being connected” means not only a direct connection but also an electrical connection. Thus, an element and a wiring may be additionally formed between objects to be connected to each other. In addition, in this specification, “overlapping” means not only the case where elements included in a display device (such as an insulating film or a wiring) are overlapped in direct contact with each other, but also the case where the elements are overlapped with each other with another element interposed therebetween.
According to the foregoing structure, current flowing through the light-emitting element in the pixel can be kept constant even if the temperature of the light-emitting element (electroluminescent layer) is changed. Accordingly, the power consumption of the display device can be prevented from increasing and luminance can also be kept constant even if the ambient temperature of the display device is increased.